This invention relates to a process and apparatus for firing an industrial furnace of the type in which at least the combustion zone is either not open to the atmosphere or substantially insulated therefrom, e.g. by a pressure difference, and which is commonly used for heating materials such as metals (e.g., a bar reheat furnace, a soaking pit, or an aluminum melting furnace), glass, etc. More particularly, this invention relates to a furnace firing method and apparatus which utilize oxygen or oxygen-enriched air as the oxidant gas instead of air.
It is common practice for air to be employed as the oxidant gas in industrial furnaces of the type described above. It is also known that oxygen enrichment of the oxidant gas for combustion, by substitution of oxygen in place of part or all of the air, can reduce the fuel requirements for and help increase the production rate of industrial furnaces. As oxygen replaces air for combustion, the nitrogen portion is correspondingly reduced in both the oxidant and the flue gas, thus reducing the total volume of each, on a per-unit-of-fuel-burned basis, and increasing the oxygen concentration of the oxidant-fuel mixture. These changes are, in turn, responsible for the following principal advantages:
(1) Increase in the maximum achievable firing rate for the burners of a given furnace, which can be used to augment production rate. With air as the oxidant, the firing rate may be limited by (a) the air that can be supplied to the burner through the available ducts and blowers, (b) the volume of combustion products that can be handled by the flue, and (c) the firing rate that can be tolerated by the burner, before combustion instability and incomplete combustion present problems. With an increase in the amount of oxygen, the lower oxidant and flue gas volumes overcome the first two limitations, while the lower oxidant volume and higher oxygen concentration help overcome the third limitation.
(2) Decrease in fuel consumption. With air as the oxidant, the sensible heat loss to the flue gas is often substantial due to the high nitrogen content of air. With oxygen enrichment, the nitrogen content of the flue gas is reduced and the heat content of the flue gas is decreased resulting in lower sensible heat losses at comparable off gas temperatures. The overall fuel savings per unit of production can be very significant.
(3) Decrease in pollution problems relating to entrainment of particulates, due to the lower flue gas volume. Gas cleaning of all pollutants is less costly and more effective with a decreased volume of flue gas per unit of fuel burned.
The extent of the above benefits increases with the degree of oxygen enrichment. Therefore, use of substantial oxygen enrichment as well as use of pure oxygen would be desirable in the art. Such use, however, has been avoided in the art to date, because it suffers from the following disadvantages:
(1) High flame temperatures. Flame temperature increases markedly as the oxygen concentration in the oxidant gas increases. This is undesirable because it results in (a) unusually high heat transfer rates in a localized region around the flame which can result in "hot spots" causing damage to the furnace refractory and/or the furnace charge, and (b) higher nitrogen oxide (NO.sub.x) emissions, as the kinetics and equilibria of the NO.sub.x formation reactions are significantly favored by high temperatures. Use of pure oxygen as the oxidant gas does not solve the second problem by limiting the availability of nitrogen, because sufficient nitrogen is usually present in the furnace, through air leaks (which are usually unavoidable, even in closed furnaces, especially in industrial scale operations) or in the fuel, to form nitrogen oxides in environmentally unacceptable quantities, i.e. in amounts exceeding the acceptable NO.sub.x emission standards.
(2) Low gas momentum in the furnace. The reduction in mass in both the oxidant and in the fuel, can result in a substantial reduction in the incoming oxidant gas and fuel jet momentum, which, in turn, reduces the amount of mixing and recirculation of the gases within the furnace. Good mixing and gas circulation in the furnace are necessary to obtain effective heat transfer and uniform heating of the charge as well as further to avoid localized hot spots.
Thus, although the aforedescribed advantages of using oxygen or oxygen-enriched air in place of air in industrial furnaces were known, such use was avoided because it was accompanied by the aforedescribed disadvantages. There exists, therefore, a need in the art for a process and apparatus for firing a furnace which permits use of oxygen or oxygen-enriched air as the oxidant gas, thereby taking advantage of the benefits such use affords, but which overcomes the disadvantages set forth above.